Lubrication Tips for Reliability Professionals

June 26, 2001 Subscribers: 7,326

Q & A

“What is the difference between absolute and kinematic viscosity?”

The absolute viscosity of an oil is defined as the resistance to flow and shear under the forces of internal friction. This internal friction is caused by the resistance of oil molecules moving relative to each other. The larger the molecules, the higher the internal resistance and consequently the higher the absolute viscosity. The absolute viscosity of an oil is usually measured using a Brookfield or Rotary Viscometer (ASTM D2983).

Kinematic viscosity on the other hand is a measure of an oils resistance to flow and shear under the forces of gravity. Again, the larger the molecules, the greater resistance, the higher the kinematic viscosity. Kinematic viscosity is usually measured using a Capillary Tube Viscometer (ASTM D445) and is the method most commonly used by oil analysis labs to determine an oils viscosity.

The absolute viscosity of an oil can be calculated from the kinematic viscosity by multiplying by the specific gravity. However, in doing so, care must be taken since the specific gravity of an oil can be affected by a number of different contaminants including water, oil oxidation by-products, soot, glycol, fuel and wear debris.

Today's Tip

• Nitration is the degradation of oil in the presence of nitrogen compounds. Nitration is a common mode of gas engine lubricant degradation. It is a particular problem with higher temperature 4-cycle engines. Nitrogen oxides are typically formed during fuel combustion. These nitric oxides react with water to form nitric acid. The formation of nitric acids can lead to a corrosive environment to exposed engine surfaces. Nitration also leads to the formation of deposits and sludge.
  

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Book Bits

Combined with a high operating temperature (above 140 deg. F), water reacts with and destroys zinc type antiwear additives. For example, zinc dithiophosphate (ZDDP) is a boundary lubricant that reduces wear in high-pressure pumps, gears and bearings. When this additive is depleted, abrasive wear accelerates rapidly. This will show up as premature component failures, resulting from metal fatigue and other wear mechanisms

For more information about the book "Filtration Technology" go to:

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